1. Field of the Invention
This invention relates to a radiation image storage panel using a stimulable phosphor, and more particularly to a radiation image storage panel for recording and reproducing a radiation image using a stimulable phosphor which stores radiation energy and emanates the energy in the form of light upon stimulation thereof.
2. Description of the Prior Art
As is well known in the art, a photographic method using a silver salt such as radiography in which an X-ray film having an emulsion layer comprising a silver salt is used in combination with an intensifying screen has generally been employed to obtain a radiation image. Recently, from the viewpoint of problems such as shortage of silver resources, a method of obtaining a radiation image without using a silver salt has been desired.
An example of such a method is disclosed in U.S. Pat. No. 3,859,527. In the method of the patent, is used a radiation image storage panel comprising a stimulable phosphor which emits light when stimulated by an electromagnetic wave selected from visible light and infrared rays after exposure to a radiation (The term "radiation" as used herein means an electromagnetic wave or a corpuscular radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, high-energy neutron rays, cathode rays, vacuum ultraviolet rays, ultraviolet rays, or the like.). The method comprises the steps of (i) causing the stimulable phosphor of the panel to absorb a radiation passing through an object, (ii) scanning the panel with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor of the panel as light emission, and (iii) electrically converting the emitted light to an image.
The radiation image storage panel employed in the above-mentioned method for recording and reproducing a radiation image has at least a fluorescent layer comprising a proper binder and a stimulable phosphor dispersed therein. Although the fluorescent layer itself can be a radiation image storage panel when the fluorescent layer is self-supporting, the fluorescent layer is generally provided on a proper substrate to form a radiation image storage panel. Further, a protective layer for physically and chemically protecting the fluorescent layer is usually provided on the exposed surface of the fluorescent layer. Furthermore, a subbing layer is sometimes provided between the fluorescent layer and the substrate to closely bond the fluorescent layer to the substrate.
In the practical use of the above-mentioned method for recording and reproducing a radiation image, needless to say, it is desirable that the radiation image storage panel employed in the method not only has a high sensitivity, but also provides an image of high sharpness. However, the sensitivity of the conventional radiation image storage panel having such a structure as mentioned above and the sharpness of the image provided by the panel are incompatible with each other. Accordingly, when the sensitivity of the panel is heightened, the sharpness of the image provided by the panel is lowered, and reversely, when the sharpness is heightened, the sensitivity is lowered. That is, the sensitivity of the radiation image storage panel can be heightened by increasing the thickness of the fluorescent layer thereof. However, the thickness of the fluorescent layer is increased in order to heighten the sensitivity, the sharpness is lowered. Reversely, the sharpness can be heightened by decreasing the thickness of the fluorescent layer. However, when the thickness of the fluorescent layer is decreased in order to heighten the sharpness, the sensitivity is lowered. Since the above-mentioned incompatibility between the sensitivity and the sharpness of the conventional radiation image storage panel is substantially large, it has been difficult to obtain a radiation image storage panel which provides both practical sensitivity and sharpness. Accordingly, a radiation image storage panel which provides an image of higher sharpness than the conventional one at practical sensitivity is desired.
As described above, when the thickness of the fluorescent layer of the radiation image storage panel is increased, the sharpness of the image provided by the panel is lowered. This is because in the method for recording and reproducing a radiation image utilizing the stimulability of a stimulable phosphor, the sharpness of the image obtained depends upon the degree of spread of the stimulating rays in the panel, and the thicker becomes the fluorescent layer, the larger becomes the spread of the stimulating rays in the fluorescent layer. That is, in the method for recording and reproducing a radiation image utilizing the stimulability of a stimulable phosphor, the radiation image stored in the panel is taken out of the panel sequentially as described above. Therefore, all of the light emission caused by the stimulating rays at a certain period (t.sub.i) is desirably detected as the output of a certain picture element (x.sub.i, y.sub.i) on the panel which is exposed to the stimulating rays during the period (t.sub.i). When the fluorescent layer of the panel is thick, and therefore the stimulating rays spread in the fluorescent layer due to scattering or the like and stimulate the phosphor surrounding the picture element (x.sub.i, y.sub.i) in addition to the picture element (x.sub.i, y.sub.i), the output for the area broader than the picture element (x.sub.i, y.sub.i) is detected as the output of the picture element (x.sub.i, y.sub.i), and therefore, the sharpness of the image obtained is lowered.